We analysed Rossi X‐ray Timing Explorer (RXTE)/PCA and HEXTE data of the transient black hole binary GX 339‐4, collected over a time‐span of 8 years. We studied the properties and the behaviour of low‐frequency quasi‐periodic oscillations (QPOs) as a function of the integrated broad‐band variability and the spectral parameters during four outbursts (2002, 2004, 2007 and 2010). Most of the QPOs could be classified following the ABC classification which has been proposed before. Our results show that the ABC classification can be extended to include spectral dependencies and that the three QPO types have indeed intrinsically different properties. In terms of the relation between QPO frequency and power‐law flux, types A and C QPOs may follow the same relation, whereas the type B QPOs trace out a very different relation. Type B QPO frequencies clearly correlate with the power‐law flux and are connected to local increases of the count rate. The frequencies of all QPOs observed in the rising phase of the 2002, 2007 and 2010 outbursts correlate with the disc flux. Our results can be interpreted within the framework of the recently proposed QPO models involving Lense–Thirring precession. We suggest that types C and A QPOs might be connected and could be interpreted as being the result of the same phenomenon observed at different stages of the outburst evolution, while a different physical process produces type B QPOs.

We present a systematic analysis of the fast time variability properties of the transient black hole binary GRO J1655-40, based on the complete set of Rossi XTE observations. We demonstrate that the frequencies of the quasi periodic oscillations and of the broad band noise components and their variations match accurately the strong field general relativistic frequencies of particle motion in the close vicinity of the innermost stable circular orbit, as predicted by the relativistic precession model.We obtain high precision measurements of the black hole mass (M = (5.31 ± 0.07) M⊙, consistent with the value from optical/NIR observations) and spin (a = 0.290 ± 0.003), through the sole use of X-ray timing.

We present a systematic study of the orbital inclination effects on black-hole transients fast time-variability properties. We have considered all the black-hole binaries that have been densely monitored by the Rossi XTE satellite. We find that the amplitude of low-frequency quasi periodic oscillations (QPOs) depends on the orbital inclination. Type-C QPOs are stronger for nearly edge-on systems (high inclination), while type-B QPOs are stronger when the accretion disk is closer to face-on (low inclination). Our results also suggest that the noise associated with type-C QPOs is consistent with being stronger for low-inclination sources, while the noise associated to type-B QPOs seems inclination independent. These results are consistent with a geometric origin of the type-C QPOs -for instance arising from relativistic precession of the inner flow within a truncated disk -while the noise would correspond to intrinsic brightness variability from mass accretion rate fluctuations in the accretion flow. The opposite behavior of type-B QPOs -stronger in low inclinations sources -supports the hypothesis that type-B QPOs are related to the jet, the power of which is the most obvious measurable parameter expected to be stronger in nearly face-on sources.

We report on the first 180 days of RXTE observations of the outburst of the black hole candidate IGR J17091-3624. This source exhibits a broad variety of complex light curve patterns including periods of strong flares alternating with quiet intervals. Similar patterns in the X-ray light curves have been seen in the (up to now) unique black hole system GRS 1915+105. In the context of the variability classes defined by Belloni et al. (2000) for GRS 1915+105, we find that IGR J17091-3624 shows the ν, ρ, α, λ, β and µ classes as well as quiet periods which resemble the χ class, all occurring at 2-60 keV count rate levels which can be 10-50 times lower than observed in GRS 1915+105. The so-called ρ class "heartbeats" occur as fast as every few seconds and as slow as ∼100 seconds, tracing a loop in the hardness-intensity diagram which resembles that previously seen in GRS 1915+105. However, while GRS 1915+105 traverses this loop clockwise, IGR J17091-3624 does so in the opposite sense. We briefly discuss our findings in the context of the models proposed for GRS 1915+105 and find that either all models requiring near Eddington luminosities for GRS 1915+105-like variability fail, or IGR J17091-3624 lies at a distance well in excess of 20 kpc or, it harbors one of the least massive black holes known (< 3M ⊙ ).

We present the root mean square (rms)–intensity diagram for black hole transients. Using observations taken with the Rossi X‐ray Timing Explorer, we study the relation between the rms amplitude of the variability and the net count rate during the 2002, 2004 and 2007 outbursts of the black hole X‐ray binary GX 339−4. We find that the rms–flux relation previously observed during the hard state in X‐ray binaries does not hold for the other states, when different relations apply. These relations can be used as a good tracer of the different accretion regimes. We identify the hard, soft and intermediate states in the rms–intensity diagram. Transitions between the different states are seen to produce marked changes in the rms–flux relation. We find that one single component is required to explain the ∼40 per cent variability observed at low count rates, whereas no or very low variability is associated to the accretion‐disc thermal component.

Accretion of matter onto black holes is universally associated with strong radiative feedback 1 and powerful outflows 2 . In particular, black hole transients 3 show outflows whose properties 4 are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disc encircling the black hole, and collimated radio jets 5,6 . Very recently, a distinct optical variability pattern has been reported in the transient black hole transient V404 Cyg, and interpreted as disrupted mass flow into the inner regions of its large accretion disc 7 . Here, we report on the discovery of a sustained outer accretion disc wind in V404 Cyg, which is unlike any seen previously. We find that the outflowing wind is neutral, has a large covering factor, expands at 1% of the speed of light and triggers a nebular phase once accretion sharply drops and the ejecta become optically thin. The large expelled mass (> l0 -8 M ¤ ) indicates that the outburst was prematurely ended when a sizeable fraction of the outer disc was depleted by the wind, detaching the inner regions from the rest of the disc. The luminous, but brief, accretion phases shown by transients with large accretion discs 2 imply that this outflow is most likely a new fundamental ingredient regulating mass accretion onto black holes.The X-ray binary V404 Cyg (GS 2023+338) is a confirmed stellar-mass BH 8 with a precisely determined distance of 2.4 kpc 9 . Following 25 years of quiescence, the Swift mission detected renewed activity on Jun 15, 2015 10 , initiating a 2-week period of intensely violently variable emission across all wavelengths 11,12 . Our high signal-to-noise GTC optical spectra covering the entire X-ray/radio active phase (~15 days) show that, contemporaneously with radio jet emission, continuous ejections of neutral material at ~0.01c are present from low-level accretion phases (<1% of the Eddington luminosity; L EDD ) to the Xray peak (Methods; Fig. 1, ED Fig. 1). These are observed in hydrogen (Balmer) and helium (He I) emission lines as deep P-Cyg profiles throughout the outburst 13 , and extremely broad wings once the Xray and radio fluxes decay. P-Cyg profiles result from resonant scattering in an expanding outflow with a spherical geometry or at least sustaining a large solid angle 14, 15 (Methods). Among a dozen transitions showing this feature, the deepest are seen in the He I-5876 emission line, which is used as a reference for this study (see ED Fig. 2.).The strongest P-Cyg profiles are witnessed during days 1 to 6 (Fig. 1 and Fig. 2 for the evolution of the profiles during day 2; Methods), when the X-ray luminosity is typically 10 3 times fainter than the ~L EDD flares displayed later in the outburst 7,11 (ED Fig.1). Blue-shifted absorptions are as deep as 30% below the continuum level and we measure terminal velocities in the range V T =1,500 -3,000 km s -1 (Fig. 1, Fig 2, ED Fig 2; ED Fig. 3). Symmetric red-shifted (i.e. positive velocity) outflow emission, completely detached from the accre...

We have systematically studied a large sample of the neutron star low mass X-ray binaries (LMXBs) monitored by the Rossi X-ray Timing Explorer (50 sources; 10000+ observations). We find that the hysteresis patterns between Compton dominated and thermal dominated states, typically observed in black hole LMXBs, are also common in neutron star systems. These patterns, which also sample intermediate states, are found when looking at the evolution of both X-ray colour and fast variability of ten systems accreting below ∼ 30% of the Eddington Luminosity (L Edd ). We show that hysteresis does not require large changes in luminosity and it is the natural form that state transitions take at these luminosities. At higher accretion rates neutron stars do not show hysteresis, and they remain in a thermal dominated, low variability state, characterized by flaring behaviour and fast colour changes. Only at luminosities close to L Edd , are high variability levels seen again, in correspondence to an increase in the fractional contribution of the Comptonization component. We compare this behaviour with that observed in LMXBs harbouring black holes, showing that the spectral, timing and multi-wavelength properties of a given source can be determined by its location in the fast variability-luminosity diagram, which, therefore, provides a common framework for neutron star and black hole accretion states.

We report results obtained from a systematic analysis of X-ray lags in a sample of black hole X-ray binaries, with the aim of assessing the presence of reverberation lags and studying their evolution during outburst. We used XMM-Newton and simultaneous RXTE observations to obtain broad-band energy coverage of both the disc and the hard X-ray Comptonization components. In most cases the detection of reverberation lags is hampered by low levels of variability signal-to-noise ratio (e.g. typically when the source is in a soft state) and/or short exposure times. The most detailed study was possible for GX 339-4 in the hard state, which allowed us to characterize the evolution of X-ray lags as a function of luminosity in a single source. Over all the sampled frequencies (∼ 0.05 − 9 Hz) we observe the hard lags intrinsic to the power law component, already well-known from previous RXTE studies. The XMM-Newton soft X-ray response allows us to detail the disc variability. At low-frequencies (long time scales) the disc component always leads the power law component. On the other hand, a soft reverberation lag (ascribable to thermal reprocessing) is always detected at high-frequencies (short time scales). The intrinsic amplitude of the reverberation lag decreases as the source luminosity and the disc-fraction increase. This suggests that the distance between the X-ray source and the region of the optically-thick disc where reprocessing occurs, gradually decreases as GX 339-4 rises in luminosity through the hard state, possibly as a consequence of reduced disc truncation.